Lignin is an amorphous three-dimensional polymer consisting of phenylpropane structures. Lignin is formed from three monolignol monomers, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which undergo enzymatic polymerization resulting in a complex polymer linked via eleven types of covalent bonds. The β-O-4 linkage is the most common bond accounting for between 30% and 60% of all linkages.
Several oxidation techniques have been investigated for the production of aromatic monomers from lignin. Alkaline nitrobenzene oxidation (ANBO) has been used to determine the lignin polymeric structures via oxidative deconstruction to component monomers. Consequently, high yields of aromatic products have been reported. (Yamamura, M., Hattori, T., Suzuki, S., Shibata, D., Umezawa, T. Plant Biotechnology 27 305-310 (2010).) However, limitations on commercial utilization for monomer production include the requirement for stoichiometric quantities of nitrobenzene, high energy use and safety concerns due to possible explosions. There are also reports of catalytic wet oxidation methods being used to produce aromatic aldehydes utilizing expensive noble metal catalysts (Salesa, F. G., Maranhaob, L. C. A., Lima Filhob, N. M., Abreu, C. A. M. Chemical Engineering Science 62 5386-5391 (2007)), less expensive metals (Wu, G. X., Heitz, M. Journal of Wood Chemistry and Technology 15 189-202 (1995)) or Perovskite-type oxides (Zhang, Z., Deng, H., Lin, L. Molecules 14 2747-2757 (2009)). The main drawbacks to these methods are the expense of catalyst production, the cost and environmental concerns associated with the catalyst recovery and the use of large quantities of caustic reagents. The Borregaard plant in Norway uses a wet oxidation process to produce vanillin from lignosulphonates and, with the exception of vanilla bean extraction, it is currently the world's only supplier of vanillin derived from natural sources. Environmental remediation requirements of the wet oxidation process, along with the limited supply of lignosulphonates as the pulping industry has moved to alternative pulping processes, have limited its viability.